1. Grease and line two 8'' sandwich tins and preheat the oven to 180°C/gas mark 4.
2. For the cake, place the chocolate, 75g of the light muscovado sugar and milk in a saucepan over a gentle heat. Stir until the chocolate and sugar have melted and leave the mixture to cool slightly.
3. Whisk the remaining sugar with the butter until pale and creamy. Gradually beat in the egg before adding the vanilla, salt and whisking in the chocolate milk. Sift the dry ingredients over the liquid batter and fold in.
4. Divide the batter between your two sandwich tins and bake for 25 - minutes, or until an inserted skewer comes out clean. Leave the cake to cool in their tins on top of a wire rack for 10 minutes before turning out to cool completely.
5. For the buttercream, whisk the butter for a few seconds until pale and creamy, before sifting over half of the icing sugar and whisking again until everything is combined.
6. Add the chocolate and whisk through before sifting over the remaining icing sugar. Whisk for a few minutes, adding a splash of milk to slightly slacken the buttercream if you need to.
7. Sandwich the cakes together with half of the buttercream and use the remaining half to spread over the top of the cake. Scatter over the chocolate flakes if using.